JP2024519009A - Multi-ply container board for use in corrugated packaging - Google Patents

Abstract

The present invention relates to a multi-layer container board for use in corrugated board, the multi-layer container board comprising a first outer ply containing a strength enhancer, a second outer ply containing a strength enhancer, and an interface layer joining the first and second outer plies, each of the first and second outer plies comprising at least 50 wt % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis, the interface layer comprising the strength enhancer in an amount of 0.5 to 20 gsm, the amount of strength enhancer in the interface layer being higher than, and preferably at least twice as high as, the concentration of the strength enhancer in each of the first and second outer plies.

Description

The present invention relates to a multi-layer container board for use in corrugated board.

Corrugated cardboard (sometimes called corrugated cardboard or corrugated fiberboard) is a packaging material that can be converted into various types of packaging solutions. Corrugated cardboard is a fibrous substrate made from cellulose fibers. The fibers can be virgin fibers or recycled fibers, such as fibers derived from post-consumer cardboard or other materials.

Corrugated board contains at least one medium (fluting) and at least one flat base paper (liner or linerboard) bonded to the surface of the medium. For example, corrugated board may consist of a layer of fluting bonded between two layers of liner to form a sandwich structure. The sandwich structure can be formed in various formats such as single, double, and triple wall, as described, for example, in Kirwan M., J., Paper and Paperboard. Packaging Technology, Blackwell Publishing 2005.

One challenge when producing corrugated board is the adhesion of the liner to the fluting. Too little adhesion can cause delamination, and adding too much adhesive to ensure adequate adhesion can cause washboarding and curling of the board. It is important that the adhesive added is optimally absorbed into the liner and/or corrugating medium. If the adhesive does not adhere to the fluting/liner, delamination will occur, and if too much adhesive adheres to the fluting/liner, the same can happen.

There are various types of corrugated board qualities, which may contain different types of liner and medium. Container board (also known as CCM or corrugated case material) is a type of paperboard manufactured specifically for the production of corrugated board and contains both linerboard and medium (or fluting), the two types of paper that make up corrugated board. Container board is generally brown in color because it is made primarily from natural unbleached wood fibers, but its color can vary depending on the type of wood, pulping process, recycling rate, and impurity content.

Examples of different types of liners are kraftliners and testliners. Kraftliners are typically produced from kraft pulp, which can be bleached or unbleached, and may contain one or more layers/plies, where the top layer/ply is often optimized to provide a good print surface and good moisture resistance. Testliners are primarily produced from recycled corrugated cardboard and are usually manufactured with two layers/plies. Due to the presence of recycled fibers, testliners may typically have lower mechanical strength than kraftliners, especially lower burst strength. Kraftliners are often used for packaging boxes where there are higher demands on strength properties.

Fluting is formed from paper or paperboard that has been corrugated using a corrugator with the aid of heat, moisture and pressure.

Fluting is often prepared from Neutral Sulfite Semi-Chemical (NSSC) pulp. NSSC pulp, usually made from hardwood species, is noted for its exceptional stiffness and high firmness, making it suitable for use in fluting. Neutral Sulfite Semi-Chemical (NSSC) pulping is an old process well known in the field of paper pulping. One of the reasons for using NSSC pulping is the high yield, usually above 60%. In NSSC pulping, the cooking liquor contains a sulfite salt, such as _Na2SO3 or ( _NH4 ) _2SO3 , and a base _, such as NaOH or _Na2CO3 . By " _neutral " it is meant that the pH of the NSSC cooking liquor is generally between 6 and _10. The pulp can be cooked in a batch or continuous cooker. Typically, cooking times are between 5 minutes and 3 hours, and cooking temperatures are between 160 and 200°C. NSSC pulps contain a relatively high amount of residual lignin, such as 15-20%, which makes them hard. The Kappa number of NSSC pulps is typically greater than 70. NSSC pulping is "semi-chemical" in the sense that it also includes mechanical refining of the pulp. Refining can be done, for example, using a disc refiner at digester pressure or atmospheric pressure.

Currently, the strength and mechanical properties of the fluting and medium are improved by adding small amounts of chemical pulp to the mechanical pulp. Typically, 5-15% chemical pulp is added, which of course increases the cost but slows down the dewatering rate. One possible method is to mix a semi-chemical pulp such as NSSC with unbleached kraft pulp, but this can cause undesirable light mottling and color changes, as well as changes in organoleptic properties.

The fluting and liner are attached to each other by placing an adhesive between the core and the liner. The liner is attached to at least one surface of the core by the adhesive. The adhesive is preferably applied to at least one surface of the fluted core and then the liner is attached to said surface. Any adhesive conventional in the art can be used. The adhesive can be, for example, a starch-based glue that can be extracted from a wide variety of plants. Some of the most common plants are corn, wheat, barley, rice, potato, tapioca and pea. The starch is preferably natural, i.e., the starch is not modified. The adhesive can also include water, sodium hydroxide and boric acid. Other additives may be added, such as additives that improve wet strength or adhesive bond strength. Also, other functional chemicals, such as borax, glyoxal or mixtures thereof, can be added, for example to improve moisture resistance or gelling behavior.

One significant challenge when making corrugated board and medium is resistance to moisture. When corrugated board is exposed to moisture, water and water vapor can diffuse through the liner and soften the medium. A common solution to this problem is to increase the basis weight of the liner and/or fluting, but this conflicts with environmental imperatives that call for lower basis weight materials that consume less raw materials.

Another solution is to provide the liner with a barrier layer to reduce water and water vapor transmission. However, this is only an incomplete solution, since moisture diffusion can still occur on the other side or through the edges, thus affecting the mechanical stability of the board. Barrier layers also increase costs and usually reduce the recyclability of the material.

The fluting or core may also be treated or coated with hydrophobizing agents, but this generally increases cost and may also have a negative effect on the mechanical properties of the fluting. High levels of hydrophobizing agents may also impair the adhesion of the fluting to the liner. In particular, NSSC pulp requires high levels of hydrophobizing agents to obtain the required level of water resistance in the finished fluting.

New machine concepts and increased machine speeds, combined with an increased desire to conserve resources, have further increased the need for pulp with improved properties.

There remains a need for new and improved fluting and liner materials that combine strength, low basis weight, water/moisture resistance, low chemical consumption, low cost, and/or high recyclability.

The object of the present disclosure is to provide an improved container board, preferably for use in corrugated board, that solves or ameliorates at least some of the problems discussed above.

A further object of the present disclosure is to provide a container paperboard having improved strength properties.

A further object of the present disclosure is to provide container board that uses strength and performance agents more efficiently.

A still further object of the present disclosure is to provide a method for producing multi-ply container board, preferably for use in corrugated board, using more efficient strength and performance agents and/or more efficient dewatering.

The above objectives, as well as other objectives that will be appreciated by one of ordinary skill in the art in light of this disclosure, are achieved by various aspects of the present disclosure.

The present invention is based on the inventive recognition that more efficient use of strength and performance agents in the manufacture of NSSC-based container board as well as more efficient dewatering can be achieved if the strength and other performance agents are provided as a separate interface layer at the interface between two NSSC-based plies. If all agents are mixed directly with the NSSC-based furnish before forming, the agent retention is poor and the fines distribution is difficult to control. The problem is also that high amounts of retention and strength agents are required. The addition of strength and other performance agents in the interface layer can improve the strength of the NSSC-based container board and can also reduce the cost of the container board by reducing the amount of agent required to obtain the properties required for the NSSC-based container board. The application of strength and other performance agents in the interface layer also allows for more rapid dewatering of the container board than when the agent is mixed with the NSSC pulp. Without being bound to any particular scientific theory, it is believed that placing the agent in the interface layer does not hinder dewatering to the same extent as when the agent is provided mixed with the NSSC pulp. It has also been found that the strength of the container board can be further improved by adding a strength enhancer to the outer plies of the container board. As a result, it has been found that the combination of an interface layer containing a high amount of strength enhancer and two outer plies also containing strength enhancer improves the strength, particularly the compressive strength, of the container board.

According to a first aspect presented herein, there is provided a multi-layer container board for use in corrugated board, the multi-layer container board comprising:
a first outer ply including a strength enhancer;
a second outer ply including a strength enhancer; and an interface layer joining the first and second outer plies;
each of the first and second outer plies comprises at least 50 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis;
A multi-ply container board is provided, wherein the interface layer comprises a strength enhancer in an amount of 0.5 to 20 gsm, the amount of strength enhancer in the interface layer being greater than, and preferably at least two times greater than, the concentration of strength enhancer in each of the first and second outer plies.

The container board of the present disclosure is a multi-ply container board that includes at least two plies, a first outer ply (also referred to as the top ply) and a second outer ply (also referred to as the back ply). The outer plies are joined by an interface layer. The outer surfaces of the multi-ply container board, i.e., the surfaces of the top and back plies away from the interface layer, are referred to as the top and back sides, respectively.

Due to the high NSSC pulp content, the container board of the present disclosure is particularly useful as a medium or fluting used in corrugated board. Thus, in a preferred embodiment, the container board is fluted.

As mentioned above, the container board of the present disclosure may be used as a liner in corrugated board for applications where a high NSSC pulp content can be tolerated.

Placing higher amounts of additives, including strength additives, in the interface layers rather than distributing them throughout the multi-ply container board may result in better glue uptake at the corrugator (due to higher starch uptake by the outer plies). Placing additives in the middle plies rather than the outer plies also allows higher amounts of additive to be used, which may result in less mechanosorptive creep in the finished board.

The container board of the present disclosure is a multi-layer container board including at least two plies joined by an interface layer. The container board can be manufactured on a paper or board machine adapted to manufacture multi-layer container board. Paper or board machines for making container board are well known in the art. Typically, the machine layout includes a stock processing section, a wet end section, a press and dry section, and optionally a calendar and/or coating section. In the wet end section, the plies may be formed separately using different headboxes and laminated wet, or may be formed together in a multi-layer headbox. If formed separately, the plies are typically laminated before the press and dry section of the paper machine.

"NSSC pulp" is obtained by "NSSC pulping" and is as such defined in the Background section. NSSC pulp can be hardwood or softwood pulp, or a mixture thereof. NSSC pulp is preferably hardwood pulp or a hardwood/softwood pulp mixture containing less than 15 wt% softwood, preferably less than 10 wt% softwood, more preferably less than 5 _wt % softwood. Hardwood can be, for example, aspen, alder, poplar, eucalyptus, birch, acacia, or beech. NSSC pulp is preferably prepared by cooking with a cooking liquor containing sulfite, preferably _Na2SO3 or ( _NH4 ) _2SO3 , and a _base , preferably _NaOH or _Na2CO3 . In some embodiments, the yield from NSSC pulping is greater than 60%, preferably greater than 65%, preferably greater than 70%, more preferably greater than 75%. The term "neutral" means that the pH of the NSSC cooking liquor is in the range of 6-10. The cooking time is preferably in the range of 5 minutes to 3 hours. The cooking temperature is preferably in the range of 160-200°C. The NSSC pulp may contain a relatively high amount of residual lignin, such as 15-20%. The Kappa number of the NSSC pulp is typically above 70 according to ISO 3260, preferably above 80, preferably above 95, more preferably above 100. NSSC pulping is "semi-chemical" in the sense that it also includes mechanical refining of the pulp. Refining may be carried out, for example, using a disc refiner at digester pressure or at atmospheric pressure. Refining may be carried out in one or more steps at the same or different pulp consistencies. The first refining step may preferably be carried out at a higher consistency, such as 5-35%, and the second refining step may preferably be carried out at a lower consistency, <5%.

In some embodiments, the NSSC pulp has a moisture retention value (WRV) in the range of 120-300%, preferably in the range of 120-270%. WRV values may be determined using 100 mesh wire according to ISO standard 23714.

Each of the first and second outer plies comprises at least 50 wt% NSSC pulp on a dry weight basis. In some embodiments, each of the first and second outer plies comprises at least 60 wt% NSSC pulp on a dry weight basis, preferably at least 70 wt% NSSC pulp. The first and second outer plies may comprise 100 wt% NSSC pulp, but more typically the plies may also comprise other components such that the first and second outer plies comprise 95 wt% or less, 90 wt% or less, 85 wt% or less, 80 wt% or less, or 75 wt% or less NSSC pulp on a dry weight basis.

The non-NSSC pulp portions of the first and second outer plies may include any type of fiber, such as hardwood and/or softwood fibers, for example, chemical pulp, mechanical pulp, thermomechanical pulp, or chemi-thermomechanical pulp (CTMP). The non-NSSC pulp portions of the first and second outer plies may also include, for example, recycled fibers. For example, the first and second outer plies of the present disclosure may consist essentially of NSSC pulp or a mixture of NSSC pulp and recycled fibers. By "recycled fibers" we mean fibrous materials that have previously been incorporated into some paper or board product. Alternatively, or as a complement, the non-NSSC pulp portions may include, for example, reject pulp. For example, the pulp of the present disclosure may consist essentially of NSSC pulp and reject pulp. By "reject pulp" we mean pulp prepared by refining screen rejects from a pulping process.

In some embodiments, each of the first and second outer plies further comprises at least 10 wt. %, preferably at least 20 wt. %, and more preferably at least 30 wt. % unbleached kraft pulp on a dry weight basis. Unbleached kraft pulp, or UBKP, generally refers to unbleached sulfate pulp based on pine and/or spruce.

In some embodiments, the basis weight of each of the first and second outer plies is in the range of 20 to 120 g/ ^m2 , preferably in the range of 30 to 80 g/ ^m2 . The total basis weight of the multi-layer container board is preferably in the range of 60 to 300 g/ ^m2 .

In some embodiments, the first and second outer plies are formed from the same pulp suspension or from pulp suspensions having the same composition. In some embodiments, the composition of the first and second outer plies is the same, or nearly the same. In some embodiments, the composition and basis weight of the first and second outer plies is the same, or nearly the same. Having the same, or nearly the same, first and second outer plies reduces problems with deformation of the multi-ply container board when exposed to fluctuations in humidity and temperature.

Due to the high NSSC pulp content in the outer plies, the multi-layer container board has a high NSSC pulp content overall. In some embodiments, the multi-layer container board comprises at least 50 wt% NSSC pulp on a dry weight basis, preferably at least 60 wt%. In some embodiments, the multi-layer container board comprises 50-95 wt% NSSC pulp on a dry weight basis, preferably at least 60-95 wt%.

In some embodiments, the NSSC pulp used in the multi-ply container board is a fractionated NSSC pulp. The fractionated NSSC pulp is obtained by size fractionation of the NSSC pulp starting material into a fine fiber fraction and a coarse fiber fraction. Compared to the starting material, the fine fiber fraction has a higher amount of shorter, finer fibers. In other words, the average particle size of the NSSC pulp in the fine fiber fraction is lower than the average particle size of the NSSC pulp in the coarse fiber fraction. The fine fiber fraction may be obtained, for example, by separating the NSSC pulp starting material with a pressure screen to obtain a fraction of shorter, finer fibers.

The fine fiber fraction obtained by size fractionation of NSSC pulp is particularly advantageous for use in the outer plies of multi-ply container boards because it has less effect on the optical properties of the liner compared to the unfractionated or coarse fiber fractions of NSSC pulp.

The interface layer is preferably formed by applying an aqueous suspension containing the strength enhancer between the first and second outer plies to obtain an interface layer between the first and second outer plies. Depending on the extent to which the aqueous suspension and the strength enhancer are absorbed into the first and second outer plies, the interface layer may take the form of a separate ply or an interface zone containing a high content of the strength enhancer at the interface between the two outer plies.

The first and second outer plies and the interface layer include at least one strength enhancer. The strength enhancer is preferably selected from the group consisting of starch-based strength enhancers, cellulose-based strength enhancers, and mixtures thereof.

In some embodiments, the strength enhancer is a starch-based strength enhancer. The starch-based strength enhancer may include, for example, cooked or gelatinized or uncooked starch, or mixtures thereof.

The strength of fiber and paperboard products can be increased by increasing fiber-fiber contact, for example by surface fibrillation. One possibility to increase the strength of coarser fiber mixtures is to add finely divided cellulose materials as strength enhancers, such as cellulose fines obtained, for example, from whitewater during web formation, highly refined cellulose, or microfibrillated cellulose (MFC).

In some embodiments, the strength enhancer is a cellulose-based strength enhancer. The cellulose-based strength enhancer preferably comprises or consists of fine cellulose material, such as highly refined cellulose. Refining, or beating, of cellulose pulp refers to the mechanical treatment and modification of cellulose fibers to impart desired properties to the fibers.

In some embodiments, the cellulose-based strength enhancer has a water retention (WRV) value of ≧250%, more preferably ≧300%. Additionally, the WRV value is preferably ≦500%, more preferably ≦450% or ≦400% or ≦350%. In some embodiments, the cellulose-based strength enhancer has a WRV value of 250-400%, or 250-380%, or 250-350%, or 300-350%. WRV values may be determined using 200 mesh wire according to ISO standard 23714.

In some embodiments, the cellulose-based strength enhancer has a Schopper-Riegler (SR) number greater than 70, preferably in the range of 70 to 98, as determined by ISO standard 5267-1.

In some embodiments, the strength enhancer is selected from the group consisting of cellulose fines, highly refined cellulose having a Schopper-Riegler number in the range of 70 to 90, microfibrillated cellulose (MFC), and mixtures thereof.

The term cellulose fines as used herein generally refers to cellulose particles that are significantly smaller in size than cellulose fibers. In some embodiments, the term fines as used herein refers to fine cellulose particles that can pass through a 200 mesh screen (76 μm equivalent hole diameter) of a conventional laboratory fractionator (SCAN-CM 66:05). There are two main types of fiber fines: primary and secondary fines. Primary fines are generated during pulping and bleaching, where they are removed from the cell wall matrix by chemical and mechanical treatments. As a result of their origin (i.e., complex intercellular layers, ray cells, parenchyma cells), primary fines exhibit a flake-like structure that shares only small amounts of small fiber material. In contrast, secondary fines are generated during the refining of the pulp.

As used herein, the term highly refined cellulose pulp refers to cellulose pulp that has been subjected to significant refining, but not enough that all of the cellulose pulp passes through a 200 mesh screen (76 μm equivalent hole diameter) of a conventional laboratory fractionator (SCAN-CM 66:05). As used herein, the term highly refined cellulose pulp refers to cellulose pulp having a Schopper-Riegler (SR) number greater than 70, preferably in the range of 70 to 90, as determined by ISO Standard 5267-1.

Microfibrillated cellulose (MFC) in this patent application means cellulose particles, fibres or fibrils having a width or diameter between 20 nm and 1000 nm.

There are various methods to make MFC, such as single or multiple pass refining, prehydrolysis followed by refining or high shear disintegration or release of fibrils. One or several pretreatment steps are usually required to make the production of MFC energy efficient and sustainable. The cellulose fibers of the pulp used in producing MFC in this way may be natural or may be enzymatically or chemically pretreated, for example to reduce the amount of hemicellulose or lignin. The cellulose fibers may be chemically modified before fibrillation, where the cellulose molecules contain functional groups other than (or additional to) those found in the original cellulose. Such groups include, among others, carboxymethyl (CM), aldehyde and/or carboxyl groups (cellulose obtained by N-oxyl mediated oxidation, e.g. "TEMPO"), or quaternary ammonium (cationic cellulose). After being modified or oxidized by one of the above methods, it is easier to break down the fibers into MFC.

MFC can be produced from wood cellulose fibres, both hardwood or softwood fibres. It can also be made from microbial sources, agricultural fibres such as wheat straw pulp, bamboo, bagasse or other non-wood fibre sources. It can be made from pulps including those derived from virgin fibres, such as mechanical, chemical and/or thermomechanical pulps. It can also be made from shredded or recycled paper.

In some embodiments, the strength enhancer comprises a mixture of a starch-based strength enhancer and a cellulose-based strength enhancer. In some embodiments, the ratio of the content of the starch-based strength enhancer to the content of the cellulose-based strength enhancer is in the range of 0.1:10 to 10:0.1, more preferably in the range of 0.1:5 to 5:0.1.

In some embodiments, the interfacial layer further comprises a crosslinking agent. The crosslinking agent may be, for example, citric acid. In some embodiments, the interfacial layer further comprises an insolubilizer. The insolubilizer may be, for example, an amino resin, glyoxal, or a zirconium salt insolubilizer.

The interface layer preferably contains strength enhancers and optionally other performance agents such as starch, retention/drainage aids, and internal sizing agents.

The interface layer may also contain some cellulose fibers, i.e., cellulose in a fibrous form that is not highly refined or microfibrillated. The interface layer preferably has a significantly lower cellulose fiber content than the first and second outer plies. The interface layer preferably contains less than 30 wt. % cellulose fibers, more preferably less than 20 wt. % cellulose fibers, based on the dry weight. In some embodiments, the interface layer contains no or very little cellulose fibers. In some embodiments, the interface layer contains less than 2 wt. % cellulose fibers, preferably less than 0.5 wt. % cellulose fibers, based on the dry weight.

The interface layer has a basis weight substantially lower than the basis weights of the first and second outer plies. In some embodiments, the interface layer has a basis weight in the range of 0.5 to 20 gsm, preferably in the range of 1 to 10 gsm.

The interface layer preferably consists essentially of the strength enhancer. In some embodiments, the interface layer comprises at least 50 wt %, preferably at least 70 wt %, of the strength enhancer on a dry weight basis.

The interface layer comprises said strength enhancer at a basis weight in the range of 0.5 to 20 gsm, preferably in the range of 1 to 10 gsm, and even more preferably in the range of 1 to 7 gsm.

The first and/or second outer ply preferably comprises a strength enhancer in an amount of 2-50 kg/tonne, preferably 5-30 kg/tonne, based on dry weight. It is preferred to add a starch based strength enhancer in an amount of 2-15 kg/tonne, preferably 4-10 kg/tonne, based on dry weight. It is preferred to add a cellulose based strength enhancer in an amount of 2-50 kg/tonne, preferably 5-40 kg/tonne, or even more preferred 10-30 kg/tonne, based on dry weight.

The first and second outer plies may contain the same type of strength enhancer, but it is also possible to use different strength enhancers in the two outer plies. The same amount of strength enhancer may be used in the first and second outer plies, but it is also possible to add different amounts of strength enhancer to the two outer plies.

The amount of strength improver in the interface layer is higher than in the outer plies at the interface layer. In some embodiments, the amount of strength improver is higher in the interface layer than in each of the first and second outer plies, preferably at least two times higher. Preferably, the amount of strength improver in the interface layer is at least 4, 6, 8 or 10 times higher than the amount of strength improver in each of the first and second outer plies. If the two outer plies contain different amounts of strength improver, the interface layer contains an amount at least two times higher than the outer ply with the highest amount of strength improver. The amount of strength improver in the interface layer, having a higher amount of strength improver in the interface layer and a lower amount in the outer plies, provides improved drainage and better retention of strength improver in the multi-layer container board. It has also been found that the compressive strength of the container board is improved by the addition of a lower amount of strength improver in the outer plies and a higher amount of strength improver in the interface layer.

The interfacial layer may further include additives such as fillers, retention and/or drainage aids, flocculating additives, deflocculating additives, dry strength additives, softening agents, crosslinking aids, sizing agents, dyes and colorants, wet strength resins, solidifying agents, defoaming aids, microbial and slime control aids, or mixtures thereof.

In some embodiments, the interface layer further comprises a retention/drainage aid at a concentration at least two times greater than the concentration of the retention/drainage aid in each of the first and second outer plies.

Placing the retention/drainage aid in the interfacial layer allows for better formation but can also reduce the total amount of retention aid required.

In some embodiments, the interface layer further comprises an internal size. In some embodiments, the interface layer further comprises an internal size at a concentration at least two times higher than the concentration of the internal size in each of the first and second outer plies.

The internal sizing agent is preferably a hydrophobizing sizing agent. In some embodiments, the internal sizing agent is selected form the group consisting of alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), rosin size, and mixtures thereof.

In some embodiments, the amount of internal sizing agent in the multi-layer container board ranges from 0.5 to 5 kg/tn.

In some embodiments, the ratio of the content of the internal sizing agent to the content of the strength enhancer is in the range of 0.1:10 to 10:0.1, more preferably in the range of 0.1:5 to 5:0.1.

It has been found to be beneficial for the multi-layer container board not to be over-dried. In particular, it has been found that better fracture toughness of the multi-layer container board is obtained when dried to a particular moisture content. In some embodiments, the moisture content of the multi-layer container board is in the range of 3-17 wt%, preferably in the range of 4-14 wt%, and more preferably in the range of 5-15 wt%.

In some embodiments, the multi-layer container board has a fracture toughness index GEOM (ISO/TS 17958) greater than 6 Jm/kg, preferably greater than 7 Jm/kg, and more preferably greater than 8 Jm/kg.

In some embodiments, the multi-layer container board has an SCT index GEOM (ISO 9895) greater than 23 Nm/g, preferably greater than 24 Nm/g, and more preferably greater than 25 Nm/g.

In some embodiments, at least one of the outer plies of the multi-layer container board is optimized to provide a good print surface and good moisture resistance. In some embodiments, at least the outer ply intended as the outer surface of the corrugated board is optimized to provide a good print surface and good moisture resistance. In some embodiments, the optimization to provide a good print surface and good moisture resistance includes surface sizing. In some embodiments, the multi-layer container board is surface sized. In some embodiments, the multi-layer container board is surface sized with starch. In some embodiments, the multi-layer container board is surface sized with a combination of starch and at least one other functional ingredient, preferably selected from the group consisting of a crosslinking agent, a strengthening agent, and a hydrophobizing sizing agent. The crosslinking agent can be, for example, citric acid. The strengthening agent can be, for example, microfibrillated cellulose (MFC). The hydrophobizing sizing agent can be, for example, alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), SMA (styrene maleic anhydride), rosin size, or a mixture thereof.

Corrugated board contains at least one layer of uncorrugated liner and at least one layer of fluting. In typical corrugated board production, the flutes are corrugated and then glued to a linerboard. For example, corrugated board may consist of a layer of fluting sandwiched between two layers of liner.

According to a second aspect shown herein, there is provided a corrugated board comprising a multi-layer container board as defined with reference to the first aspect as fluting and/or a liner.

Multi-layer containerboard used for corrugating:
a) forming a first web layer from a first pulp suspension containing a strength enhancing agent and dewatering the first web layer to obtain a first outer ply;
b) applying an aqueous suspension containing a strength enhancing agent to obtain an interface layer on the first outer ply;
c) forming a second web layer from a second pulp suspension containing a strength enhancing agent and dewatering said second web layer to obtain a second outer ply on the interface layer,
wherein each of the first and second pulp suspensions comprises at least 50 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis;
The aqueous suspension comprises a strength enhancer in an amount of 0.5 to 20 gsm, the amount of strength enhancer in the interface layer being higher than the concentration of strength enhancer in the first and second pulp suspensions, preferably at least two times higher.

The terms first and second web layer do not necessarily refer to the order in which the web layers are formed. The web layers can be formed simultaneously or individually in any order.

In some embodiments, the first and second web layers are formed separately using different headboxes and one or more wires, partially dewatered, and then laminated in a wet state. An interface layer is provided before the first and second web layers are laminated.

In some embodiments, the first web layer, the interface layer, and the second web layer are formed together and partially dewatered using a multi-layer headbox and a single wire.

In some embodiments, each of the first and second pulp suspensions contains at least 60 wt. % NSSC pulp on a dry weight basis, preferably at least 70 wt. % NSSC pulp.

In some embodiments, each of the first and second pulp suspensions further comprises at least 10 wt. %, preferably at least 20 wt. %, and more preferably at least 30 wt. % unbleached kraft pulp on a dry weight basis.

In some embodiments, the basis weight of each of the first outer ply and the second outer ply is in the range of 20 to 100 g/m ² , preferably in the range of 30 to 80 g/m ² .

In some embodiments, the compositions of the first and second pulp suspensions are the same. In some embodiments, the compositions and basis weights of the first and second outer plies are the same or nearly the same.

In some embodiments, the aqueous suspension contains less than 2 wt. % cellulose fibers, preferably less than 0.5 wt. %, based on dry weight.

In some embodiments, the aqueous suspension is applied as an intermediate layer using a multi-layer headbox.

The process includes forming and dewatering a multi-ply web from a pulp suspension. Methods of forming and dewatering a web having multiple layers are well known in the art. The container board can be made on a paper or board machine adapted to produce multi-ply container board. Paper or board machines for making multi-ply container board are well known in the art. Typically, the machine layout includes a stock processing section, a wet end section, a press and drying section, and a calender and/or coating section.

The web is generally formed and dewatered in a wet end section containing one or more wires as is conventional in the art. The outer plies may be formed separately and partially dewatered using different headboxes and laminated wet, or may be formed together in a multi-layer headbox. The web is typically formed on a gap former, but may also be formed on a Fourdrinier-type former. If formed separately, the wet plies are typically laminated or couched together prior to the press and dryer sections of the papermaking machine.

The aqueous suspension containing the strength enhancer is applied between the first and second outer plies. In some embodiments, the aqueous suspension is formed as a separate aqueous layer between the first and second outer plies using a multi-layer headbox. The aqueous suspension may also be applied between the first and second outer plies using a non-contact deposition technique such as spray or foam or curtain coating application before the plies are laminated or couched together. Preferably, the solids content of the aqueous dispersion is in the range of 0.5 to 50 wt %, more preferably in the range of 1 to 30 wt %.

The web is typically subjected to further dewatering, for example the formed multi-layer web may be passed through a press section of a paper machine where the web passes between large rolls under high pressure to squeeze out as much water as possible. The press section may consist of a traditional nip press unit and a press woven felt, and/or further one or several shoe presses or extended dewatering nips. These may operate at various nip or press loads including various positions, temperatures and delay times. The press section may comprise one or more shoe presses to maximize production. If one or several shoe presses are used, these may operate at pressure levels of more than 800 kN/m, such as more than 1000 kN/m, such as more than 1200 kN/m, or such as more than 1450 kN/m. The removed water is typically received by a fabric or felt.

After the press section, the multi-layer web may be subjected to drying in a drying section. Drying may include, for example, drying the multi-layer web by passing the multi-layer web around a series of heated drying cylinders. Drying may typically remove the moisture content to a level of about 1-15 wt %. However, it has been found that the multi-layer container board benefits from not being over-dried. In particular, it has been found that better fracture toughness of the multi-layer container board is obtained when dried to a particular moisture content. In some embodiments, the moisture content of the multi-layer container board is in the range of 3-17 wt %, preferably in the range of 4-14 wt %, and more preferably in the range of 5-15 wt %.

The web can be further conditioned with heat and steam and fed between large corrugated rolls to give the finished fluting its corrugated shape.

While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is not intended that the invention be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, and the invention is intended to include all embodiments falling within the scope of the appended claims.

The examples show that high amounts of NSSC pulp in the outer plies combined with interfacial layers containing strength enhancers can be used to provide good mechanical properties to multi-ply container board.

Multi-layer container board preparation process A multi-layer container board comprising a front ply (corresponding to the first outer ply of the present invention), a back ply (corresponding to the second outer ply of the present invention) and an interface layer containing starch as a strength enhancer was prepared on a pilot machine equipped with three headboxes and three wires, a press section and a dryer section. The target final moisture content for the three-ply construction was 8.5 wt% and the target basis weight was 120 g/ ^m2 . The web was calendered on-line with a machine calender at 100°C and a nip load of 45 kN/m.

The NSSC pulp had an Lc(w) fiber length of 1.14 mm as determined by the FS5 ISO method. The FS5 curl was 2% and the FS5 fibrillation was 1.55%. The FS5 Fines A content was 20.3%.

Unbleached kraft pulp (UBKP) was refined to SR30. The corresponding fiber properties were: Lc(w) fiber length 2.32 mm, FS5 curl 6.8%, FS5 fibrillation 1.78%. FS5 Fines A content was 18.8%.

All pulp suspensions contained the same amounts of retention and drainage aids and strength and internal sizing agents (AKD).

The pH of the pulp suspension was approximately 7.0 (±0.5).

When the starch was applied between the outer plies, the application rate was about 2 g/ ^m2 on a dry weight basis. The starch was applied by spraying an aqueous suspension containing the starch onto the back ply and then the plies were couched together prior to the press section.

Example 1 (Comparative Example)
The 120 g/m ² 1-ply container board shown in Table 1 was prepared from a pulp suspension containing 92.5% NSSC pulp and 7.5% UBKP according to the multi-layer container board preparation method.

The resulting multi-layer container board was analyzed. The results are shown in Table 2.

Examples 2A and 2B
The pulp suspension of Example 1 was split into two different batches and run in a multi-layer wet forming mode with two headboxes and two wires as shown in Table 1. The webs were couched together before the press section. Sample 2A was coated with 2 g/ ^m2 starch and Sample 2B was not coated with starch.

The resulting multi-layer container board was analyzed. The results are shown in Table 2.

Examples 3A and 3B
A corresponding multi-layer structure was made as made in Example 2, but using a 50-50 pulp mixture of NSSC and UBKP for the top ply and a 95-5 pulp mixture of NSSC and UBKP for the back ply, as described in Table 1. In Sample 3A, 2 g/ ^m2 starch was applied between the two webs, and in Sample 3B, no starch was applied.

The resulting multi-layer container board was analyzed. The results are shown in Table 2.

Examples 4A and 4B (Comparative)
A corresponding multi-layer construction was made as made in Example 2, but using 100% UBKP for both the top and back plies as shown in Table 1. In Sample 4A, 2 g/ ^m2 starch was applied between the two webs, and in Sample 4B, no starch was applied.

The resulting multi-ply container board was analyzed and the results are shown in Table 2.

Unless otherwise specified, the physical properties discussed in this disclosure are determined according to the following standards:
Whiteness C/2°+UV ISO 2470-1
L*C/2°+UV ISO 5631-1
a*C/2°+UV ISO 5631-1
b* C/2°+UV ISO 5631-1
Basis weight ISO 536
Thickness, single sheet ISO 534
Umbrella, single sheet ISO 534
Air permeability G-H ISO 5636-5
Cobb 30s ISO 535
Moisture content 50% rh ISO 287
Scott-Bond TAPPI T569
Tensile strength ISO 1924-3
Tensile Index ISO 1924-3
Tensile strength md/cd ISO 1924-3
Elongation ISO 1924-3
Tensile stiffness ISO 1924-3
Tensile stiffness index ISO 1924-3
E-Module of Elasticity ISO 1924-3
TEA ISO 1924-3
TEA Index ISO 1924-3
TEA Index ISO 1924-3
Fracture toughness ISO/TS 17958
Fracture toughness index ISO/TS 17958
Tear resistance ISO 1974
Tear index ISO 1974
SCT ISO 9895
SCT Index ISO 9895
RCT ISO 12192
RCT Index ISO 12192
Burst index ISO 2759
Bursting strength ISO 2759

Unless otherwise specified, standard methods may be applied to determine the physical and mechanical properties in both the cross direction (cd) and machine direction (md).

Claims (15)

1. A multi-layer container board for use in corrugated board, said multi-layer container board comprising:
a first outer ply including a strength enhancer;
a second outer ply including a strength enhancer; and an interface layer joining the first and second outer plies;
each of the first and second outer plies comprises at least 50 wt. % neutral sulfite semi-chemical (NSSC) pulp on a dry weight basis;
1. A multi-ply container board, wherein the interface layer comprises a strength enhancer in an amount of 0.5 to 20 gsm, the amount of strength enhancer in the interface layer being higher than, and preferably at least two times higher than, the concentration of strength enhancer in each of the first and second outer plies. The multi-layer container paperboard of claim 1, wherein each of the first and second outer plies comprises at least 60 wt. %, preferably at least 70 wt. %, NSSC pulp on a dry weight basis. The multi-layer container paperboard of claim 1 or 2, wherein each of the first and second outer plies further comprises at least 10 wt. %, preferably at least 20 wt. %, more preferably at least 30 wt. %, unbleached kraft pulp on a dry weight basis. 4. The multi-ply container paperboard according to any one of claims 1 to 3, wherein the basis weight of each of the first outer ply and the second outer ply is in the range of 20 to 120 g/ ^m2 , preferably in the range of 30 to 80 g/ ^m2 . The multi-layer container board of any one of claims 1 to 4, wherein the first and second outer plies are formed from pulp suspensions having the same composition. The multi-layer container paperboard of any one of claims 1 to 5, wherein the interface layer has a basis weight in the range of 0.5 to 20 gsm. The multi-layer container paperboard of any one of claims 1 to 6, wherein the interface layer comprises at least 50 wt. %, preferably at least 70 wt. %, of the strength enhancer on a dry weight basis. The multi-layer container paperboard of any one of claims 1 to 7, wherein the interface layer comprises the strength enhancer at a basis weight in the range of 1 to 10 gsm, preferably in the range of 1 to 7 gsm. The multi-layer container paperboard of any one of claims 1 to 8, wherein the strength enhancer is selected from the group consisting of starch-based strength enhancers, cellulose-based strength enhancers, and mixtures thereof. The multi-layer container paperboard of any one of claims 1 to 9, wherein the strength enhancer is a cellulose-based strength enhancer selected from the group consisting of cellulose fines, highly refined cellulose having a Schopper-Riegler number in the range of 70 to 90, and microfibrillated cellulose (MFC). The multi-layer container paperboard of any one of claims 1 to 10, wherein the interface layer further comprises a retention/drainage aid at a concentration at least twice as high as the concentration of the retention/drainage aid in each of the first and second outer plies. The multi-layer container paperboard of any one of claims 1 to 11, wherein the interface layer further comprises an internal size at a concentration at least twice as high as the concentration of the internal size in each of the first and second outer plies. A multi-layer container board according to any one of claims 1 to 12, wherein the first and second outer plies contain strength enhancers in an amount of 2 to 50 kg/tonne, preferably 5 to 30 kg/tonne. A corrugated board comprising the multi-layer container board of any one of claims 1 to 13 as fluting and/or a liner. The corrugated board of claim 14, which includes multi-layer container board as fluting.